Nervous system function depends on the construction of complex, ordered synaptic connections among neurons and targets during development. At the Drosophila larval neuromuscular junction (NMJ), and many other synapses, neural activity regulates cAMP levels through Ca2+ regulation of adenylyl cyclases (AC), leading to reductions in the levels of the homophilic cell adhesion molecules like FASII that act by restraining synaptic growth; thus down regulation of FASII through the activity-mediated increase in synaptic cAMP is necessary for structural expansion of the synapse. However, Ca+2 regulated ACs are important coincidence detectors, integrating increases in neuronal Ca+2 with the activation of transmembrane receptors coupled to the stimulation of ACs through the heterotrimeric G protein, Gs. To test the idea that receptor signaling through s plays a role in synaptic growth, we have taken advantage of the fact that all receptor-mediated pathways or activation of ACs require the a subunit of the Gs complex (Gsa). Consistent with a role for Gsa signaling, we have shown that the Gsa protein is concentrated in growing synaptic boutons and that mutations in the gene encoding Gsa inhibit neuronal arborization and bouton formation, leading to deficits in sensory-motor processes as assayed on both a behavioral and physiological level. Furthermore, restricted expression of Gsa indicates that Gsa pathways are likely involved in the reciprocal interactions between pre- and postsynaptic cells required for the growth and development of mature synapses. These and other preliminary results suggest that Gsa-dependent regulation of AC activity plays an important role during processes that underlie synaptic growth and plasticity. To further test this hypothesis, this proposal focuses on the following three specific aims:1. Comprehensive Assessment of the NMJ Phenotypes Generated by dgs Mutations.In order to investigate the formation of NMJ in hypomorphic dgs mutants at higher resolution, we will carefully quantify ultrastructural defects generated by these mutations at the E.M. level and use of electrophysiological approaches to determine if altered synaptic morphology is accompanied by altered synaptic transmission. In addition, to test our working hypothesis that hypomorphic dgs mutations result in attenuated, but not eliminated, signaling through Gsa, we will examine phenotypes generated by mutations in additional residues in the C terminus of Gsa and directly assess receptor-mediated signaling though individual mutant Gsa by biochemical approaches.2. Test Models of the Role of Gsa Signaling in NMJ Formation through Genetic Epistasis.The object of this aim is to test our revised model of NMJ formation by examining the functional relationship between processes defined by specific mutations through genetic epistasis. Our focus will be on mutations which have been used to develop existing models, since the utility of this approach has already been demonstrated (e.g., mutations affecting neuronal activity, cAMP, cell adhesion). We will use these studies to precisely define the role of receptor-dependent activation of adenylyl cyclase through Gsa within the context of each tissue (neurons and muscle) in the establishment and growth of synaptic connections at the larval NMJ.3. Identify Molecules that Participate in Pathways Activated By Gsa during NMJ Formation.A major advantage of the Drosophila NMJ is that powerful genetic approaches can be applied in the identification of molecules that participate in Gsa-regulation of synaptic plasticity. Thus, the goal of this aim is to identify participants in the Gsa-activated pathway operating specifically during NMJ formation through the use of the genetic interaction strategies possible in Drosophila.